The present invention relates to the art of antennas, and more particularly to a slot antenna having particular application in the field of UHF broadcasting.
Slotted cylinder antennas have been used effectively in UHF broadcasting since the early 1950's. These antennas remain the most popular transmitting antennas for these purposes because of the high gain associated therewith, as well as the simplicity of the feeding arrangement required and their ability to generate desired pattern shapes.
Prior art slotted cylinder antennas have been arranged and dimensioned so that the electromagnetic energy propagates along the interior of the radiating cylinder in either the TEM mode or the TM.sub.01 mode. One of the reasons that these modes have been selected in the past is that they exhibit cylindrical symmetry. The exact position of the slots about the circumference of the radiating cylinder is, therefore, not critical. Also, although the current lines in the TEM mode are entirely longitudinal, horizontal polarization can still be generated with relative simplicity (in a vertically oriented slotted cylinder) through the provision of longitudinally extending slots, as long as a suitable coupling device (such as shown in Bazan U.S. Pat. No. 2,981,947) is provided for each slot. This is desirable because horizontal polarization is currently the accepted standard for UHF broadcasting.
There is disclosed herein, however, a slotted cylinder antenna wherein the selected mode of propagation within the radiating cylinder is the TE.sub.11 coaxial mode. A number of advantages inhere in the use of this mode of propagation. The use of this mode allows the construction of a UHF antenna having a relatively small diameter, thereby providing a low windload of the antenna. Additionally, it is possible to center-feed an antenna employing this mode so as to thereby secure the many benefits associated with a center fed antenna. Yet another advantage of the use of this mode is that the transmission of circularly polarized signals may be easily provided for, merely through the provision of pairs of orthogonal slots at selected positions along the antenna. This is possible because the TE.sub.11 mode includes both longitudinal and transverse current components. This is particularly important in view of the growing acceptance of circularly polarized antennas as a desirable alternative to conventional horizontally polarized antennas.
A problem associated with the use of the TE.sub.11 mode relates to the non-symmetrical nature of the current lines about the antenna axis. Because of this lack of cylindrical symmetry, the relative positions of the mode and the slots is critical to the attainment of a predictable radiation characteristic. The orientation of the mode may wander within a coaxial waveguide of conventional construction, however, due to conductor imperfections, manufacturing tolerences, or discontinuities within the system. Consequently, the desired alignment between the slots and the mode is not readily achievable in these waveguides.
The present invention resolves this problem by structuring the coaxial waveguide so that a preferred field orientation exists. When structured thusly, the TE.sub.11 mode will be fixed in a known orientation. It is therefore possible to position the slots in any desired alignment with respect to the mode polarization.
In accordance with the present invention, a slotted cylinder antenna is provided including a radiating structure which is constructed and dimensioned so that the electromagnetic energy will propagate therethrough in the TE.sub.11 mode, and so that the mode polarization will remain fixed in a known orientation. The radiating structure is periodically interrupted by radiating slots which are positioned along the radiating structure so that the desired transmission polarization and radiation pattern are secured.
In accordance with another feature of the present invention, the radiating structure comprises a cylinder having a coaxial feed line disposed therein. The coaxial feed line runs along the center of the cylinder to the vicinity of the midpoint of the antenna, where a feed point is provided for exciting the TE.sub.11 mode of propagation along the waveguide defined by the exterior of the coaxial feed line and the cylinder. Electrically conductive members are disposed at selected circumferential positions within the waveguide so as to establish a preferred field orientation for the TE.sub.11 mode. Slots are provided at regular positions along the cylinder so as to radiate electromagnetic energy therefrom having a desired polarization sense and radiation pattern.
In accordance with yet another aspect of the present invention, orthogonal slots are provided along the radiating cylinder so that the polarization transmitted by the antenna is substantially circular or elliptical.